Changes in phytoplankton community structure during wind-induced fall bloom on the central Chukchi shelf

Abstract

The recent increasing of atmospheric turbulence has had considerable impact on the oceanic environment and ecosystems of the Arctic. To understand its effect on phytoplankton community structure, a Eulerian fixed-point observation (FPO) was conducted on the Chukchi shelf in fall 2013. Temporal and vertical distributions of the phytoplankton community were inferred from algal pigment signatures. A strong wind event (SWE) occurred during the observation term, and significant convection supplied nutrients from the bottom layer to the surface. Before the SWE, pigment composition in the warmer, less saline, and nutrient-poor surface waters was diverse with low concentration of chlorophyll-a (chla). Vertical mixing induced by the SWE weakened the stratification and brought sufficient nutrients to enhance diatom-derived pigment concentrations (e.g., fucoxanthin and chlc3), suggesting increases in diatoms. We also developed a model to predict the distribution of major phytoplankton pigment/chla ratios using a profiling multi-wavelength fluorometer (Multi-Exciter) with higher spatio-temporal resolution. The Multi-Exciter also captured changes in pigment composition with environmental changes at the FPO site and at four observation sites 16 km from the location of the FPO. Furthermore, we investigated the change in grazing rates of the major Arctic copepod Calanus glacialis copepodid stage five to assess the interaction between primary and secondary producers during the fall bloom. Increased diatom biomass caused a significant increase in the grazing rate on microphytoplankton (> 20 µm) and a decrease on nanophytoplankton (2-20 µm), indicative of a strong cascade effect because of the reduction of microzooplankton due to the grazing from C. glacialis. We conclude that SWEs during fall might affect food webs via the alternation of seasonal succession of phytoplankton community structure.

Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification

“…The waters in the halocline layer are laterally transported from the edge of the shelf to the central Arctic Ocean (Aagaard et al, ). The surface environment, and especially phytoplankton abundance, could change during transport (e.g., Fujiwara et al, ; Nishino et al, ) and possibly alters the light environment in the upper and middle halocline layer, which would regulate nitrification.…”

Impacts of Temperature, CO2, and Salinity on Phytoplankton Community Composition in the Western Arctic Ocean

“…After that, nutrients (NO 3 : 7 µmol L −1 , PO 4 : 0.5 µmol L −1 , and Si(OH) 4 : 5 µmol L −1 , in final concentrations) were added to all carboys prior to homogenizing the seawater. Nutrients were added to mask the effects of nutrient limitation and to mimic wind-induced nutrient upwelling, which is frequently observed in autumn along the shelf edge in the Chukchi Sea and Canada Basin, where sea ice has retreated during the summer and autumn (Nishino et al, 2015;Ardyna et al, 2017;Fujiwara et al, 2018). Note that although the added nutrient stock solutions were prepared using Milli-Q water, the amount of the additions was very small compared to that of the incubation bag (<0.1% in v/v), and the change in salinity should be consistent in all bags.…”

“…Seawater samples for HPLC were collected onto GF/F filters under a gentle vacuum, flash frozen in liquid nitrogen and stored in a deep freezer at −80 • C until laboratory analysis on land. Accessory pigments on the filter were extracted using N, N-dimethylformamide, and sonication, and extracted samples were measured by HPLC following the method of van Heukelem and Thomas (Van Heukelem and Thomas, 2001;Fujiwara et al, 2014Fujiwara et al, , 2018. We analyzed the data of the following nine pigments: peridinin (autotrophic dinoflagellate), fucoxanthin (e.g., diatoms and pelagophytes), 19 -butanoyloxyfucoxanthin (e.g., pelagophytes), 19 -hexanoyloxyfucoxanthin (e.g., haptophytes), chlorophyllb, prasinoxanthin, and violaxanthin (e.g., chlorophytes and prasinophytes), alloxanthin (e.g., cryptophytes), and zeaxanthin (e.g., cyanobacteria) (Van Heukelem and Thomas, 2001;Tremblay et al, 2009;Fujiwara et al, 2014).…”

“…Although previous studies successfully analyzed algal classes with pigment composition in the western Arctic Ocean (Fujiwara et al, 2014(Fujiwara et al, , 2018, a detailed analysis of algal pigments inhabiting the Arctic may be needed in the future. Samples for microscopic observations were collected at the beginning, during the exponential growth phase and at the end of the MR15exp., and the samples were collected at the beginning and at the end of the experiment in the MR16-exp.…”

“…Marine phytoplankton, forming the base of the marine ecosystem, have already been affected by changes in the Arctic. A shorter sea ice season as a result of earlier sea ice retreat and later freeze-up results in increases in the exposure of the sea surface to sunlight and wind stress, which has altered the productivity and phenology of phytoplankton blooms (Arrigo and van Dijken, 2011;Ardyna et al, 2014;Nishino et al, 2015;Fujiwara et al, 2018). In the Canada Basin of the western Arctic Ocean, the composition of pico-sized (<2 µm in diameter) phytoplankton tended to increase with time, probably due to low nutrient availability in strongly stratified surface waters under warmer and fresher conditions (Li et al, 2009;Ardyna et al, 2017).…”